Evaluation of seismic site effects in Lombardia (Italy) by numerical analyses for urban planning

Many field reports on past seismic events have pointed out that the effects due to some geologic and geomorphologic situations have produced different damage to similar buildings. To obtain a qualitative and quantitative classification of these situations, different qualitative and quantitative approaches have been used. In the paper the application of a methodology for the evaluation of site effects in an area of Lombardia Region (Italy) is shown. During the project 15 localities were analyzed. In this paper the results of some representative situations are presented. The objective of the project was to develop a methodology that may be easily repeated, to take into account site effects in urban planning. Therefore, a series of geologic, geomorphologic (1:5 000 scale) and geotechnical analyses have been carried out, to identify the areas affected by site effects and to characterize the lithotechnical units. The expected seismic inputs have been calculated and numerical analyses using one- dimensional (1D) code, analyzing single soil columns, and two-dimensional (2D) codes, working with finite or boundary elements on sections, have been done. The results, in terms of elastic pseudo-acceleration spectra and amplification coefficients, as a ratio between spectral intensity calculated using the pseudo-velocity spectra, in the periods of 0.1-0.5s and 0.5-1.5s of output and input, give elements for planning, both for urban general choices and for building design. The results of all the analyzed situations, provide some correlations between the geometric and geotechnical characteristics of the sites and the values of the amplification coefficients. These correlations could be used as a tool for extrapolations in similar situations, in the same area, when only the geologic, geophysical and geotechnical characteristics of a site are known. Our results showed amplification for all three lithologies considered: 'silt with clay' being the greatest and 'Gravel' the least, with significant differences seen between the two shear wave velocity - depth profiles used in this study.